Summary

Chimaeric conceptuses have been produced by injection of 5½- and 6½-day extra-embryonic ectoderm and 5½-day embryonic and extra-embryonic endoderm into 3½-day mouse blastocysts. Extra-embryonic ectoderm cells contributed only to the ectoplacental cone and/or trophoblast giant cell fractions, reflecting the probable trophectoderm origin of these cells. Proximal (visceral) endoderm cells overlying both the embryonic and extra-embryonic ectoderm contributed cells only to the endoderm of the visceral yolk sac, indicating that the definitive embryonic endoderm has not formed by 5½ days p.c.